1. Field of the Invention
This invention relates to magnetic recording elements and processes for preparing such elements. In one of its aspects, this invention relates to magnetic recording elements which combine low modulation noise with high magnetic particle alignment and orientation in the plane of the magnetic layers and processes for preparing such elements. In still another of its aspects, this invention relates to a magnetic recording medium that contains discrete, non-solid voids that are nonmagnetizable, for example air voids, which have been used to provide a recording medium in which the magnetic particles are highly aligned and oriented. Such medium exhibits excellent high frequency output and very low modulation noise in comparison to comparable commercially available prior art materials.
2. Description Relative to the Prior Art
A conventional magnetic recording element comprises a support such as a plastic film, a metal or glass substrate, and a magnetic recording medium containing acicular ferro- or ferrimagnetic particles dispersed in a binder which is commonly a nonmagnetizable polymeric material. As used herein, the term magnetic recording medium is not restricted solely to a medium composed of one or more magnetic recording layers which contain magnetic particles, but can also include the presence of other layers that are coated on the same surface of the substrate with the magnetic recording layer or layers to form a layer composite. Typical of such layers are subbing layers that are used to cause magnetic layers to adhere to the surface of substrates. The magnetic coating which generally constitutes a substantial portion of the magnetic recording medium is commonly applied to the substrate as a dispersion of the ferro- or ferrimagnetic particles in a lacquer comprising the binder in a solvent. The dispersion is applied to a substrate by mechanical means such as extrusion, dipping, screen printing and other molding or casting processes. The solvent is then removed from the coated layer by evaporation or other means, generally using a drying oven.
In the prior art it has generally been considered to be desirable to eliminate, as far as possible, voids, i.e., regions that contain substantially no magnetic recording material, from a magnetic recording medium and especially from those areas in direct association with magnetic pigment. This was a result of the fact that such regions were considered to seriously reduce electrical output of the magnetic medium and to produce output noise signals which have been characterized as modulation noise. Thus, in U.S. Pat. No. 3,240,621, issued Mar. 15, 1966, it is suggested that a desirable recording material requires that the voids content for a magnetic recording layer containing magnetizable particles, be reduced to less than 5 percent, by volume, as compared to a theoretical "void free" coating. See, for example, Col. 2, lines 23-32 of U.S. Pat. No. 3,240,621 which states, "As compared with a theoretically perfect coating containing no voids, the coatings of this invention have less than 10% voids when the magnetic particles are non-oriented (i.e., when no specific steps have been taken to orient the particles in parallel directions), and the coatings have less than about 5% voids when the particles are magnetically oriented in accordance with prior art teachings. These new coatings provide substantially greater total signal voltage output than is obtainable from present commercial coatings."